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>Appendix F. Additional Supplied Modules</TD
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><H1
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><A
NAME="INTAGG"
>F.17. intagg</A
></H1
><P
>  The <TT
CLASS="FILENAME"
>intagg</TT
> module provides an integer aggregator and an
  enumerator.  <TT
CLASS="FILENAME"
>intagg</TT
> is now obsolete, because there
  are built-in functions that provide a superset of its capabilities.
  However, the module is still provided as a compatibility wrapper around
  the built-in functions.
 </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN132439"
>F.17.1. Functions</A
></H2
><P
>  The aggregator is an aggregate function
  <CODE
CLASS="FUNCTION"
>int_array_aggregate(integer)</CODE
>
  that produces an integer array
  containing exactly the integers it is fed.
  This is a wrapper around <CODE
CLASS="FUNCTION"
>array_agg</CODE
>,
  which does the same thing for any array type.
 </P
><P
>  The enumerator is a function
  <CODE
CLASS="FUNCTION"
>int_array_enum(integer[])</CODE
>
  that returns <TT
CLASS="TYPE"
>setof integer</TT
>.  It is essentially the reverse
  operation of the aggregator: given an array of integers, expand it
  into a set of rows.  This is a wrapper around <CODE
CLASS="FUNCTION"
>unnest</CODE
>,
  which does the same thing for any array type.
 </P
></DIV
><DIV
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><H2
CLASS="SECT2"
><A
NAME="AEN132448"
>F.17.2. Sample Uses</A
></H2
><P
>   Many database systems have the notion of a one to many table. Such a table
   usually sits between two indexed tables, for example:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE left (id INT PRIMARY KEY, ...);
CREATE TABLE right (id INT PRIMARY KEY, ...);
CREATE TABLE one_to_many(left INT REFERENCES left, right INT REFERENCES right);</PRE
><P>

  It is typically used like this:

</P><PRE
CLASS="PROGRAMLISTING"
>SELECT right.* from right JOIN one_to_many ON (right.id = one_to_many.right)
  WHERE one_to_many.left = <TT
CLASS="REPLACEABLE"
><I
>item</I
></TT
>;</PRE
><P>

  This will return all the items in the right hand table for an entry
  in the left hand table. This is a very common construct in SQL.
 </P
><P
>  Now, this methodology can be cumbersome with a very large number of
  entries in the <TT
CLASS="STRUCTNAME"
>one_to_many</TT
> table.  Often,
  a join like this would result in an index scan
  and a fetch for each right hand entry in the table for a particular
  left hand entry. If you have a very dynamic system, there is not much you
  can do. However, if you have some data which is fairly static, you can
  create a summary table with the aggregator.

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE summary AS
  SELECT left, int_array_aggregate(right) AS right
  FROM one_to_many
  GROUP BY left;</PRE
><P>

  This will create a table with one row per left item, and an array
  of right items. Now this is pretty useless without some way of using
  the array; that's why there is an array enumerator.  You can do

</P><PRE
CLASS="PROGRAMLISTING"
>SELECT left, int_array_enum(right) FROM summary WHERE left = <TT
CLASS="REPLACEABLE"
><I
>item</I
></TT
>;</PRE
><P>

  The above query using <CODE
CLASS="FUNCTION"
>int_array_enum</CODE
> produces the same results
  as

</P><PRE
CLASS="PROGRAMLISTING"
>SELECT left, right FROM one_to_many WHERE left = <TT
CLASS="REPLACEABLE"
><I
>item</I
></TT
>;</PRE
><P>

  The difference is that the query against the summary table has to get
  only one row from the table, whereas the direct query against
  <TT
CLASS="STRUCTNAME"
>one_to_many</TT
> must index scan and fetch a row for each entry.
 </P
><P
>  On one system, an <TT
CLASS="COMMAND"
>EXPLAIN</TT
> showed a query with a cost of 8488 was
  reduced to a cost of 329.  The original query was a join involving the
  <TT
CLASS="STRUCTNAME"
>one_to_many</TT
> table, which was replaced by:

</P><PRE
CLASS="PROGRAMLISTING"
>SELECT right, count(right) FROM
  ( SELECT left, int_array_enum(right) AS right
    FROM summary JOIN (SELECT left FROM left_table WHERE left = <TT
CLASS="REPLACEABLE"
><I
>item</I
></TT
>) AS lefts
         ON (summary.left = lefts.left)
  ) AS list
  GROUP BY right
  ORDER BY count DESC;</PRE
><P>
 </P
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